The present invention relates to a switching power supply that supplies electricity required by a load device to the load device by switching switching elements, and to a method for stopping supply of electricity from the switching power supply to the load device.
As a type of switching power supply, DC-AC inverters are known that permit a vehicle battery to be used as a power supply for household electric appliances. Electrical components in such a switching power supply include passive components the temperature of which does not increase when the switching power supply is used with electricity less than or equal to the rated electricity level and increases when the switching power supply is used with electricity that exceeds the rated electricity level.
Japanese Laid-Open Patent Publication No. 2003-14552 discloses a temperature detecting device for preventing an electric motor and a power transistor from being damaged while operating at an overload. Specifically, the temperature detecting device accurately detects the operating temperature of a heat-producing portion of the electric motor without a temperature sensor attached to that portion. In the temperature detecting device, a first portion at which the temperature is detected is set, and a second portion is set in the vicinity of the first portion. Also, a third portion is set at a position in the vicinity of the first portion. The third portion is farther from the first portion than the second portion is from the first portion. A temperature sensor is provided in the third portion. The operation temperature T1<k>; at the first portion is successively calculated using the following three expressions.T1′<k>=(1/C1′)×{P<k>−(1/R1′)×(T1′<k−1>−T2′<k−1>)}×Δt+T1′<k−1>T2′<k>=(1/C2′)×{(1/R1′)×(T1′<k−1>−T2′<k −1 >)−(1/R2′)×T2′<k−1>}×Δt+T2′<k−1>T1<k>=T1′<k>+Tm<k>
R1′: the thermal resistance constant of a portion extending between and including the first portion to the second portion
R2′: the thermal resistance constant of a portion extending between and including the second portion to the third portion
C1′: the thermal capacity constant of a portion extending between and including the first portion to the second portion
C2′: the thermal capacity constant of a portion extending between and including the second portion to the third portion
P<k>: estimated heat value
Tm<k>: output of temperature sensor
T1′<k>: difference between the operation temperature and the temperature sensor output
T1′<k−1>: difference between the operation temperature and the temperature sensor output at a time Δt before
T2′<k>: difference between the temperature at the second portion and the temperature sensor output
T2′<k−1>: difference between the temperature at the second portion and the temperature sensor output at a time Δt before
In the above described temperature detecting device, the temperature sensor is located at a position that is a little separated from the portion where the temperature should be detected. Therefore, even if the environmental temperature changes, the change is compensated for by the output of the temperature sensor. The temperature detecting device can be applied to an apparatus used in a greatly changing temperature environment. Also, since the temperature at the portion where the temperature should be detected and the output of the temperature sensor is calculated, the range of a modeled thermal equivalent network can be reduced. This reduces errors in temperature calculations. However, in the temperature detecting device, the operation temperature T1<k> needs to be successively calculated using the three expressions at every predetermined interval, which adds to the processing load. Also, errors can accumulate. Further, in the temperature detecting device, the four constants, which are the thermal capacity constants and the thermal resistance constants, need to be accurately and separately identified.